eUTRAN also referred to as LTE is a radio communications network standardized by 3GPP. The LTE radio base station provides communication services to mobile terminals over a common frequency carrier. Use of the frequency carrier is shared among the active mobile terminals in a cell supported by the radio base station. In LTE the radio base station is named eNodeB and the mobile terminal is named User Equipment UE.
A principle for LTE is that the radio communication with a UE is performed over one radio base station, as is contrary to systems that employ soft handover. The radio access technology is OFDM, which supports communication with plural UE at the same time with separate sub-carriers of the frequency carrier temporary assigned to the different UE. Timing of the uplink transmission from the UE is carefully controlled by the eNodeB because interference between the UE can only be avoided when the signals from the different UE are received time aligned at the radio base station.
FFT (Fast Fourier Transformation) processors are very well suited for receiving OFDM signals, and, albeit also other type of processors for Fourier processing may be used, the FFT processor is one reason why the OFDM technology has been chosen for the LTE as well as for other radio communication systems such as Wimax. One FFT processor receives time domain signal samples of the frequency carrier and transforms it into frequency domains samples. In the frequency domain it is easy to filter sub-groups of sub-carriers that have been used by different UE for transmitting information to the radio base station. The radio base station can thereby easily separate the frequency domain samples from different UE and then process the separate signals such as making channel estimation for each of them.
In cells controlled by different radio base station it is intended to have a frequency reuse of one, meaning neighbouring cells will use the same frequency carrier. Communication is sometimes difficult when the UE is located at the cell edge. The uplink, UL, direction, from the UE to the radio base station is the most troublesome owing to the UE transmit power is limited. Moreover, for small cells such as micro, pico or femto cells that are covered by a larger cell, the UE power is intentionally limited in order decrease the interference caused in the overlapping cell. In discussions on the scope of 3GPP release number 10 it has been proposed that a radio base station may request a neighbour base station to assist in receiving the signals from a UE on the cell edge. The assisting neighbour base station shall then provide the requesting radio base station with information received over the air from the UE on the cell edge, such as frequency domain samples from the Fourier processing of the frequency carrier as produced when detecting the UE in its own cell. This works well when the radio base station are located at a site distance up to about 500 m, because then the UE signals received whether produced in the own cell or in the cell controlled by the close distance radio base station will be received time aligned and be possible to detect within the frequency domain samples. Signals from UE at larger distance in other cells will mostly arrive non-aligned with the signals from UE in the own cell and then just appear as interference in the frequency domain samples and will not be detectable. Communication with UE on the cell border is nevertheless troublesome also when the radio base station has no neighbour at close distance.